Relationship between porosity, pore parameters and properties of microarc oxidation film on AZ91D magnesium alloy

Microarc oxidation films on AZ91D magnesium alloy were prepared in sodium silicate-based electrolyte by a high-frequency bipolar pulsing mode. The effects of porosity and pore parameters on the properties of the films were studied. The results show that with the increased current frequency, the relative content of Mg element and the relative content of MgSiO3 phase in the films increase. The bonding strength of the films increases and the film microhardness decreases with the increased porosity respectively. Pore diameter could also affect the film properties to some degree. As the current frequency increases, the pore diameter and the film porosity decrease while the interpore distance and the pore circularity increase. The larger pore diameter and the higher porosity could promote the cracking behavior of the films while the larger interpore distance and the higher pore circularity could hinder the cracking behavior. Crack density is a better parameter than crack width when evaluating heat resistance. With decreased film porosity, the film impedance in 3.5% NaCl solution increases. The lower porosity and the better continuity of the films improve the anti-corrosion ability of the films.